**3.2. Two-tier architecture**

Two-tier architectures supply a basic network between a client and a server. For example, the basic web model is a two-tier architecture as illustrated in **Figure 3**. A web browser makes a request from a web server, which then processes the request and returns the desired response, in this case, web pages. This approach improves scalability and divides the user interface

**Figure 1.** Client server paradigm.

**Figure 2.** Single-tier architecture.

**Figure 3.** Two-tier architecture.

from the data layers. However, it does not divide application layers so they can be utilized separately. This makes them difficult to update and not specialized. The entire application must be updated because layers are not separated.

Like an individual running server is one tier and several running servers may also be counted as one tier. Layer software implementation has many advantages and is a good way to achieve N-tier architecture. Layer and tier may or may not exactly match each other. Each layer may run

Evolution and Paradigm Shift in Distributed System Architecture

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N-tier implies more than three levels or tiers involved as depicted in **Figure 5**; mostly additional tiers are associated with business logic tier. Some layers in 3-tier can be broken further into more layers. These broken layers may be able to run in more tiers. For example, application layer can be broken into business layer, persistence layer or more. Presentation layer can be broken into client layer and client presenter layer [12]. So, in order to claim a complete N-tier architecture, client presenter layer, business layer and data layer should be able to run

**a.** Client tier: This tier is involved with users directly. There may be several different types of

**b.** Client presenter tier: This contains the presentation logic needed by clients, such as

**c.** Business tier: It handles and encapsulates all of business domains and logics; also called

**d.** Persistence tier: This tier handles the read/write of the business data to the data layer, also

in an individual tier. However, multiple layers may also be able to run in one tier.

clients coexisting, such as WPF, Window form, HTML web page and etc.

in three separate computers (tiers).

**Figure 4.** Three-tier architecture.

ASP. NET MVC in IIS web server.

called data access layer (DAL).

**e.** Data tier: It is the external data source, such as a database.

as domain layer.

**Figure 5.** N-tier architecture.
